UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

pH value, which is typical of the metal-like adsorption behavior of adsorbing species (Benjamin
and Leckie, 1981). These data also showed that the adsorption edges for Pu(V) was shifted
about 2 pH units higher as compared to the adsorption edges observed for Pu(V), indicating that
plutonium in the higher oxidation state (pentavalent) had lower adsorbing affinity as compared
with tetravalent plutonium. This difference in adsorption was attributed to the fact that Pu(V)
hydrolyzes less strongly than Pu(IV),
The Pu(IV) and Pu(V) adsorption data obtained in 0.1 M NaNO3 media represents conditions
where only free cations and the respective hydrolytic species are the adsorbing species. Extensive
experimental observations have shown that, when present, strong complexing agents have a
significant effect on the metal ion adsorption (Benjamin and Leckie, 1981). This modified
adsorption behavior in the presence of complex-forming ligands is characterized by Benjamin and
Leckie as ligand-like adsorption. Sanchez et al. (1985) also conducted experiments to examine
the effect of dissolved carbonate (from 10 to 1,000 meq/l) on the adsorption of Pu(IV) and Pu(V)
on goethite. Their adsorption data showed that at a fixed pH value of 8.6, increasing carbonate
concentration beyond 100 meq/l greatly decreased the adsorption of plutonium in both oxidation
states. These data demonstrated that practically no Pu(IV) or Pu(V) adsorption occurred on
goethite when the total carbonate concentration approached 1,000 meq/l (0.5 M CO3). However,
data collected by Glover et al. (1976) showed that, at very low concentrations of dissolved
carbonate (i.e., 0.1-6 meq/l) typically encountered in soils, adsorption of Pu(IV) increased with
increasing dissolved carbonate concentration. These results indicate that Pu(IV) in these soils
3+
may adsorb in the form of PuHCO3 species.
Such complete suppression of Pu(IV) and Pu(V) adsorption was attributed to the presence of
anionic plutonium-hydroxy carbonate species in solution and to the fact that goethite at this pH
contains mainly negatively charged sites that have negligible affinity to adsorb anionic species.
This adsorption behavior of Pu(IV) and Pu(V) in the presence of carbonate ions that form strong
hydroxy carbonate complexes is typical of ligand-like adsorption of metal ions described by
Benjamin and Leckie (1981). Ligand-like adsorption is described as adsorption of a metal-ligand
complex that is analogous to adsorption of the free ligand species. Also, the metal-ligand
complexes may not adsorb at all if these complexes are highly stable. These data clearly
demonstrate that increasing total carbonate and hydroxyl solution concentrations significantly
decrease Pu(IV) and Pu(V) on iron oxyhydroxide surfaces.
Similar suppression of adsorption of higher valence state actinides in the presence of carbonate
and hydroxyl ions has been observed by a number of investigators. Some of these studies include
adsorption of U(VI) on goethite (Hsi and Langmuir, 1985; Koehler et al., 1992; Tripathi, 1984),
ferrihydrite (Payne et al., 1992), and clinoptilolite (Pabalan and Turner, 1992), and Np(V)
adsorption on ferrihydrite, hematite, and kaolinite (Koehler et al., 1992).
Some of the early plutonium adsorption experiments on soils were conducted by Rhodes (1957)
and Prout (1958). Rhodes (1957) conducted plutonium adsorption experiments using a
calcareous subsurface soil from Hanford as the adsorbent. The data indicated that adsorption
varied as a function of pH ranging from 18 ml/g under highly acidic conditions to >1980 ml/g at
G.5